Field of the Invention
The invention relates to an apparatus for transferring structures, a method for fabricating it and its use in the field of semiconductor technology.
In the last-mentioned field, the miniaturization of dimensions has an ever more important part to play. Work is being carried out to effect size reduction primarily in the case of memory components, in order to achieve a higher storage density. In the course of fabricating such memory components, photolithographic exposures for imaging the desired structures are of crucial importance. In particular in order to be able to achieve structure widths of below 0.25 .mu.m, the resolution of conventional exposure methods must be considerably improved.
According to the contemporary prior art (see e.g. "Silicon Processing for the VLSI Era", Volume 1--Process Technology, S. Wolf, R. N. Tauber, Lattice Press, Sunset Beach, Calif., USA) projection masks are used for example for fabricating structures having a width of 0.35 .mu.m. The projection masks being transferred to a wafer coated with photoresist after having their size reduced by a factor of 5 or 10 by a stepper. The resolution is in this case limited by the limits prescribed by wave optics. Diffraction phenomena and the Abbe criterion for resolution may be mentioned here. In conventional optical lithography, it is not possible to produce structures whose size is smaller than approximately the wavelength of the light used for exposure.
In order to improve the resolution and enable structure widths of below 0.25 .mu.m to be achieved, two solution paths have been proposed in principle.
On the one hand, light of a shorter wavelength is used in order to achieve an improved resolution. An example that may be mentioned is the use of excimer lasers with a wavelength of 248, 195 or 157 nm (see e.g. "Nanolithography And Its Prospects As A Manufacturing Technology", R. F. W. Pease, J. Vac. Sci. Technol. B 10(1) 1992, 278-284). This solution approach requires a considerable outlay on equipment (excimer laser with special optics) and is correspondingly expensive. In addition, it is necessary to use special photoresists having high sensitivity to the wavelength used.
A further alternative for improving the resolution consists in the use of phase masks which lead to phase shifts by use of a targeted combination of the phase rotations of light during passage through optically transparent or partly absorbing media having different thicknesses. The constructive and destructive interference caused by the phase shifts leads to imaging having improved resolution.
However, the fabrication of such phase masks requires an extraordinarily high computational outlay in order to draft the fundamental configuration of the masks. In addition, the masks have to be optimized further by experimental findings. Thus, this way of improving the resolution is also time-consuming and expensive.
A more recent development enabling the optical resolution to be improved is the scanning probe technique, which makes use for example of STM technology (STM=Scanning Tunneling Microscopy). This technique is described for example in the reference titled "Patterning Of An Electron Beam Resist With A Scanning Tunneling Microscope Operating In Air", K. Kragler, E. Gunther, R. Leuschner, G. Falk, H. von Seggern, G. Saemann-Ischenko, Thin Solid Films 264 (1995) 259-263, and in the article by R. F. W. Pease already mentioned.
The reference titled "Near-Field Optics: Light For The World Of Nano-Scale Science", D. W. Pohl, Thin Solid Films 264 (1995) 250-254, describes how the Abbe resolution limit is overcome by optical scanning near-field microscopy. In this case, a specimen surface is scanned using a thin tip having a very small optical aperture that is guided across the specimen surface with a small gap between them. The tip is generally a glass fiber on which metal has been vapor deposited. The resolution that can be achieved depends on the optical diameter of the aperture and the distance from the specimen and can be improved to below .lambda./20. For this purpose, however, it is necessary to accurately set the gap between the aperture and the specimen surface to a few nm, in order to ensure working in the optical near field. With the use of glass fibers, shear force detection is suitable for monitoring the distance.
Glass fiber tips of this type have also been used for exposing a structure. In this case, the tip is guided across the substrate to be exposed, e.g. a photoresist, and the latter is locally exposed. The structure widths that can be achieved are approximately 80 nm in air ("Scanning Near-Field Optical Lithography" (SNOL), S. Wegscheider, A. Kirsch, J. Mlynek, G. Krausch, Thin Solid Films 264 (1995) 264-267) and 50 nm in a vacuum ("Optical Near-Field Lithography On Hydrogen-Passivated Silicon Surfaces", S. Madsen, M. Mullenborn, K. Birkelund, F. Grey; Appl. Phys. Lett. 69 (4) (1996) 544-546). Further improvements in the resolution by improving the manufacturing method for the glass fiber tips are conceivable.
If it were attempted to expose a specimen surface using a single tip of this type, this would be extremely time-consuming and would not, therefore, be considered for continuous operation in the field of semiconductor technology. Moreover, it would be necessary to monitor the spacing by use of shear force detection in this case as well.
Moreover, the manufacturing process for the glass fiber tips is very poorly reproducible, with the result that corresponding tips having a defined optical aperture can only be obtained with difficulty. Consequently, tip arrays containing a plurality of tips can scarcely be used. Such arrays would additionally make it necessary to control the feeding of light to each of the tips separately and, moreover, to set the distance between each individual tip and the specimen surface separately. The outlay on equipment for this would be enormous.
Admittedly, a simplified method for manufacturing tips for scanning near-field microscopy is disclosed in the reference by W. Noell, M. Abraham, K. Mayr, A. Ruf, J. Barenz, O. Hollrichter, O. Marti and P. Guthner; titled "Micromachined Aperture Probe Tip for Multifunctional Scanning Probe Microscopy", Appl. Phys. Lett. 70 (1997) 1236-1238, and in Published, Non-Prosecuted German Patent Application DE 195 09 903 A. The tips are etched out of a silicon nitride layer with the aid of thin film technology. The problems described above in connection with the individual tips or tip arrays nonetheless remain.